cellForItem is not called when reloadData

1. To fix the flash? lightening bug when scrolling the search results I changed reloadData when paging to “insertItems”
2. After that, when I need to reloadData from SearchResults to SearchHistory, cellForItem is not called for section 0
3. The reason was realodData is called before insertItems finished
4. So I made the orders guaranteed using dispatchGroup.

private let insertItemsAndReloadDataGroup = DispatchGroup()

... after getting paged results ....
self.insertItemsAndReloadDataGroup.enter()
             self.collectionView.performBatchUpdates {
                 self.collectionView.insertItems(at: indexPaths)
             } completion: { _ in
                 self.insertItemsAndReloadDataGroup.leave()
             }

... when I go back to searchHistory from searchResult ...
insertItemsAndReloadDataGroup.notify(queue: .main) { [weak self] in
                 guard let self = self else { return }
                 self.reloadDataAndResetScroll()

5. After that I had other problems. Because I used different numbers of sections according to the types of search results. So I changed the code to reloadData when it is not paging, I mean it’s first page.

The order of viewWillDisappear and viewDidLoad when pushViewController

When I push B view controller on A view controller,

The order of viewWillDisappear and viewDidLoad when pushViewController

on iOS 10: A’s viewWillDisappear -> B’s viewDidLoad

on iOS 11+: B’s viewDidLoad -> A’s viewWillDisappear

How iOS app is launched

• main function is called. The only thing to mention about the main function is that its job is to hand control off to the UIKit framework. UIKit provides the main run loop needed to manage interactions among the user, the system, and your app
• The UIApplicationMain function(Creates the application object and the application delegate and sets up the event cycle.- main event loop) handles process 1
  1. by creating the core objects of your app
  2. loading your app’s user interface from the available storyboard files, calling your custom code so that you have a chance to do some initial setup,
  3. and putting the app’s run loop in motion. (the app frameworks automatically set up and run the run loop on the main thread as part of the application startup process.)
• During startup, the UIApplicationMain function sets up several key objects and starts the app running. At the heart of every iOS app is the UIApplication object, whose job is to facilitate the interactions between the system and other objects in the app.

 

• Not Running -Launch-> Inactive -> Active
• Swift는 @UIApplicationMain이 시작진입점이다. UIApplicationMainfunction을 호출하는 것과 같다. https://docs.swift.org/swift-book/ReferenceManual/Attributes.html -UIApplicationMain
• AppDelegate의 didFinishLaunching
• 종료:applicationWillTerminate이 호출되고 시스템이 앱을 메모리에서 제거합니다.

• main function of main.m and  UIApplicationMain function never returns.
• UIApplicationMain:
  • application: instantiates the application object from the principal class and
  • instantiates the delegate (if any) from the given class and
  • application delegate: sets the delegate for the application.
  • sets up the event cycle: It also sets up the main event loop, including the application’s run loop, and begins processing events.
  • If the application’s Info.plist file specifies a main nib file to be loaded, by including the NSMainNibFile key and a valid nib file name for the value, this function loads that nib file.

notes for paging

the filter information has to be kept ex. category

 

about URLRequest configuration

urlRequest.cachePolicy = .reloadIgnoringLocalCacheData

        urlRequest.httpShouldHandleCookies = false

To configure a session, you use a URLSessionConfiguration object, which controls behavior like how to use caches and cookies, or whether to allow connections on a cellular network.

In some cases, the policies defined in this configuration may be overridden by policies specified by an NSURLRequest object provided for a task. Any policy specified on the request object is respected unless the session’s policy is more restrictive.

When UIImageJPEGRepresentation returns nil

fileprivate lazy var ciContext = CIContext()

guard let cgImage = self.ciContext.createCGImage(outputImage, from: outputImage.extent) else {

                return

           }

            let uiImage = UIImage(cgImage: cgImage)

            guard let image = UIImageJPEGRepresentation(uiImage, 0.9) else {

                return

                

            }

try not to make UIImage directly from ciImage

1. convert ciImage to cgImage
2. then convert the cgImage to UIImage

viewController requestData

when url request failed, and when the app did become active (even after going to control tower which bottom up popup), it’s better to requestData if the url request failed.

OS

If an app does not launch within a finite amount of time, the operating system terminates the application.

app status, app lifecycle

https://developer.apple.com/documentation/uikit/uiapplicationdelegate

inactive: The app is running in the foreground but is not receiving events.

Background: The app is executing code but is not visible onscreen.

For example, the system may wake up an app so that it can process background downloads, certain types of location events, remote notifications, and other types of events.

An app in the background state should do as little work as possible. Apps that request time to process specific types of events should process those events and return control back to the system as quickly as possible.

https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/StrategiesforHandlingAppStateTransitions/StrategiesforHandlingAppStateTransitions.html#//apple_ref/doc/uid/TP40007072-CH8-SW1

https://developer.apple.com/library/archive/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html